Blood tests are often checked before starting an antiepileptic drug (AED). Blood tests may be done again after you have been taking the seizure medicine for a while, especially if there’s been a change in dose or if you have a change in how you feel.

What do the blood tests measure?

Blood tests may be done to check how much medicine is in your body, usually called drug or AED levels. (AED stands for antiepileptic drugs)

Blood tests may also be used to see how your body is tolerating the medicine. The types of blood test will vary depending on what possible side effects may be seen with your seizure medicine. These blood tests may include levels of electrolytes (substances in the body such as sodium and potassium), kidneys and liver function, and blood cell counts (such as white blood cells, red blood cells, platelet counts).

How often are blood tests needed?

How often blood tests are done depends on each seizure medicine, the need for monitoring, and other health problems you have or medicines you take. The frequency of testing also varies from doctor to doctor and even country to country.

The frequency of blood tests to check for side effects depends on the medicine you take. Many of the older seizure medicines need more monitoring than the newer ones.

Blood tests may be done before starting a drug then periodically once you are on the medicine. Some medicines may need blood tests every few weeks or months when starting the medicine. After a person is used to the medicine, these blood tests may be needed only once in a while or if problems develop. Other medicines, especially some of the newer ones, need blood test monitoring much less often if at all.

Blood tests to measure the amount of medicine in your system can also vary. Again, blood levels of the older medicines are done more often to help guide changes in doses. Well-established ranges for blood levels are available for the older drugs, but not as much for newer seizures medicines. For some of the newer seizure medicines, tests for blood levels may not be available or helpful in adjusting doses of a medicine.

What does the therapeutic range of blood levels refer to?

The so-called therapeutic range of blood levels for seizure medications is the range of drug in the body where most patients have good seizure control and few or no side effects. The lower and upper limits of this range can vary between different laboratories and different drugs.

When is the best time to test blood levels of a drug?

Usually blood levels are checked once a medicine has reached a stable amount in the body (called a ‘steady state’). This means that the amount of drug that is taken each day has to equal the amount of medicine that leaves the body (by being broken down and eliminated by the kidneys or liver).

Before checking a drug level, you should be on it long enough to reach a steady state in your body. The length of time will differ for the drugs.

Generally it takes at least 1 to 3 weeks for the drug level to be stable in the body.

Blood tests should be checked at a consistent time. This means checking it at about the same time of day, and length of time since you took the last dose of medicine. This way the results can be compared over time.

To check a steady state of drug, it’s best to check a ‘trough level’. This is the lowest amount of medicine in the body during the day. Usually it’s best to check the blood first thing in the morning before you have taken the first dose. If this can’t be done, checking a blood level at least 8 hours from the last dose can be done.

I you want to see if symptoms are related to side effects of a medicine, checking the blood when the symptoms occur is usually best. This is often about 1 to 4 or 6 hours after a dose is taken.

Is a blood drug level the main way to tell if a medicine is effective or helpful?

No. The best way to tell if a medicine is working is to talk to you and your family. How often seizures occur and if you have any side effects that bother you is often more important than a blood test. This information comes from you and your family, not a test.

The blood level helps the doctor judge if the medicine dose is high enough, too high, or if it may be time for a change. It’s a piece of information that is looked at together with how you feel. The doctor or nurse will also want to do a ‘neuro exam’ that can help pick up side effects of medicines. So remember:

The blood level is only a guide. Some people will do well at low levels of medicine. Other people may need higher amounts.

Results of a blood level need to be looked together with other information.